1. Field of the Invention
The present invention relates to porous material made of polytetrafluoroethylene (hereinafter, PTFE) which can be used as membranes for filters, diaphragms for batteries or electrolytic cells as well as covering tapes for electric cables or the like. More particularly, it relates to porous material of PTFE having finer pore size, improved permeability and dimensional stability.
The present invention also relates to a process for producing such porous material.
2. Description of the Related Art
Porous material of PTFE are used in a wide variety of industrial fields as partitions or diaphragms for fuel cells, membranes for filters, covering tapes for electric cables, membranes for analyzers or dialysis units, artificial blood vessels or the like. Recently, porous material of PTFE having finer pore size and improved permeability and dimensional stability are demanded in the applications of microfiltration, supports for high-functional membranes, air-filters or the like.
A variety of processes for producing the porous material of PTFE have been proposed as following:
(1) In Japanese patent publication No. 42-13,560, a non-sintered molded article which is produced by paste extrusion of PTFE is stretched at a temperature below a melting point of PTFE and then is sintered.
(2) In Japanese patent publication No. 53-42,794, a sintered molded article of PTFE is cooled down slowly to increase its crystallinity and then is stretched mono-directionally at a stretch ratio between 1.5 times and 4 times.
(3) Japanese patent laid-open No. 58-145,735 discloses a process comprising extruding a paste of fine powder of PTFE to prepare a non-sintered molded article, subjecting the molded article to heat-treatment at a temperature below a melting point of the PTFE fine powder but above a melting point of a shaped (sintered) product thereof under such conditions that an endothermic peak of the fine powder does not change on a differential scanning calorimeter chart (a crystal melting chart) and that the specific gravity of the shaped product becomes above 2.0, and then stretching the molded article at a temperature below the melting point of the powder.
(4) In Japanese patent laid-open No. 64-78,823, a molded article produced by paste extrusion of PTFE fine powder having number average molecular weight of lower than 1,000,000 is sintered and then is heat-treated to increase the crystallinity. After then, the resulting molded article is stretched along at least one direction.
However, in the case of the process (1) in which a non-sintered molded article is stretched, it is difficult to obtain porous material having finer pore sizes and improved in permeability. In the process (2) in which a sintered molded article is stretched, it is difficult to increase the stretch ratio, resulting in that a film obtained shows relatively lower porosity and hence lower permeability although the film obtained has fine pore sizes. In the case of process (3) in which stretching is effected after the heat-treatment, the pore size can be reduced but is not yet satisfactory and the heat-resistance is also insufficient. Relatively higher porosity as well as finer pore size can be realized in the process (4) in which a sintered molded article of PTFE whose number average molecular weight is lower than 1,000,000 is stretched. This process, however, is not sufficient to increase the permeability because it is difficult to increase the stretch ratio.
Therefore, the prior arts are not satisfactory to obtain such porous articles of PTFE as having finer pore size, higher porosity and improved heat-resistance. Still more, porous articles of PTFE produced by the prior arts are very shrinkable after they are dipped in solvents (solvent shrinkage). Such big shrinkage become a serious problem in some applications such as filters for organic solvent vapors because the filters shrink along a direction of thickness so that a gas flow rate is reduced.
Present inventors tried to solve the problems of the prior arts and found that the above-mentioned problems can be solved by subjecting a non-sintered molded article prepared by paste extruding of PTFE fine powder to such a heat-treatment condition that the resulting heat-treated molded article shows at least one endothermic peak which appears between an endothermic peak position of the fine powder (about 347.degree. C.) and an endothermic peak position of a sintered product thereof (about 327.degree. C.) on a differential scanning calorimeter chart (DSC chart), and then by stretching the heat-treated molded article along at least one direction.
This process looks like similar to the process disclosed in Japanese patent laid-open No. 58-145,735 in which the heat-treatment of a shaped product is effected before stretching. The later heat-treatment, however, is basically different from the present invention because the heat-treatment of this patent is effected under such a condition that no change of endothermic peaks of the fine powder occur on a DSC chart. In fact, in this patent, a substantially non-sintered molded article is stretched, so that the resulting porous material show relatively larger pore size, big thermal- and solvent-shrinkage, poor heat-resistance and insufficient dimensional stability. To the contrary, in the process according to the present invention, a molded article is changed to a partially sintered article (half-sintered article) by the heat-treatment and then is stretched, so that the resulting product has finer pore size, higher permeability, improved heat-resistance and dimensional stability. The present invention was completed on the basis of these findings.
Therefore, an object of the present invention is to provide porous material of PTFE having finer pore size, higher porosity and improved permeability.
Another object of the present invention is to provide porous material of PTFE which exhibit reduced heat-shrinkage and good heat-resistance (thermal stability).
Still another object of the present invention is to provide porous material of PTFE which exhibit reduced solvent shrinkage and improved dimensional stability.